Small point - can you contact the gym, explain that due to temporary circumstances you are unable to use your 10-week membership right now and can it be frozen/deferred? It's in the gym's interest to keep you happy so that you might come back/sign up for a full annual membership, so they may be accommodating.
For the rest - as has been pointed out already, your health must be paramount. If all this extra running around for others has impacted your health (and I think it may well have) then you need to consider - what will all these people do if I am so ill I can do nothing for them? Well - they'd have to make other arrangements.
I suggest you tell your daughters that right now, with their dad laid up, you need to cut back on what you do for them. And yes, point out that it's impacting your health, the GP has called you in. For starters, knock the GC's swimming lessons on the head. They can learn later, it's too much right now. Next - stop thinking the house has to be 'visitor ready'. Because what that really means is that you don't want people to see that you're struggling. Stop that, it's what's got you to this point <hug>. Can your DDs/DS do anything for their granddad to take it off your hands? I'll bet they could, if you asked. And finally, for the longer term, your eldest daughter's work hours have changed. If that's permanent she needs to alter her childcare to suit, not have you pick it up.
Christmas - you have three adult children, at least two of whom have children. Perhaps it is time for one of them to host? I first hosted the family Christmas when my wonderful MIL was ill one year. She was better the next year and wanted to host again, so they wouldn't have to commit to taking on the host role permanently, just this year when both their parents are poorly. Worth thinking about?
You need to start considering yourself first. Try to think of yourself as the first-aider helping them out. The first thing a first-aider has to consider, before ANYTHING else, is; is this environment safe for me to enter? Because the first-aider is no help to the wounded if they walk in and get wounded too. This current 'environment' is making you ill, so you need to prioritise that before giving aid.